A conventional DRAM memory cell structure includes only a single MOS transistor and associated capacitor forming, as is well known, a diffused twin cell in a semiconductor substrate. In this case, when the conventional DRAM memory cell is formed on the SOI(silicon on insulator) substrate, unwanted charges are accumulated in the bulk substrate, thus altering the electrical properties of the bulk substrate. As a result, a floating body phenomenon, which changes the threshold voltage, occurs and the unit cell does not operate uniformly.